It is known to check, statically or dynamically (i.e. with the part being stationary or in rotation), the concentricity of different cross-sections of mechanical parts with symmetry of rotation, in particular shafts, with reference to a geometrical axis defined by a totally mechanical manner.
The geometrical axis is normally defined by two precision centres (with relevant support devices) that cooperate with the corresponding seats obtained at the ends of the shaft for permitting support and rotation of the latter during the subsequent machining phases (turning, grinding, etc.). One of the centres can be motorized, for performing dynamical checkings.
In several cases, from a theoretical point of view it is more correct to take as a reference the geometrical axis defined by the centre seats, but it is difficult and expensive to make high precision centres and relevant support devices, for preventing displacements of said geometrical axis with respect to the gauging devices or heads that perform dimensional measurements in correspondence with the different cross-sections of the shaft.
Therefore, in other cases the checking is performed by taking as a reference a geometrical axis defined as the axis passing through the centres of two cross-sections of the part. For example, for a crankshaft, the reference axis can be determined by processing the signals of two gauging heads each of which has two feelers in contact with a respective journal for determining two radial dimensions of the journal along the same diametral direction.
The signals of these gauging heads are combined with those of further gauging heads that check other radial dimensions of the part. The part can be supported by Vee supporting devices or also by low precision centres, since possible spatial displacements of the axis of instantaneous rotation of the part are electronically compensated.
However, as already mentioned, sometimes the measurement problems require to take as a reference the axis defined by the seats for the supporting centres, in particular for inter-operational checkings of the machining process.